Recently, even in a field of production printing, digitization of sheet-fed printing presses and printers for continuous slip/form paper have been promoted, and many products of an electrophotograph type, an inkjet type, etc., have been introduced commercially. Demands on high quality and color reproduction stability of printed images by these apparatuses become higher.
In order to improve the color reproduction stability, a calibration of the apparatus is performed by measuring colors of the printed material with a spectrometer, or a printing condition is controlled by feeding back color information. According to the spectrometer of prior art, the measurement is performed at a point in a region of interest to print an arbitrary color chart, and then color measuring is performed based on the color chart. It is desirable that these techniques are applied to the image as a whole in order to compensate for variations of the image within a page or between pages. There are products which perform measurements at plural points by moving a single spectrometer; however, it is difficult to apply such a technique to the in-line measurements for high-speed apparatuses used for production printing, etc.
Japanese Laid-open Patent Publication No. 2005-315883 discloses a technique for the in-line color measurement over a full width of the image in which the image is illuminated over its full width with a light source having different frequency bands and the reflected light is acquired to obtain a spectral characteristic over the full width. However, according to the technique disclosed in Japanese Laid-open Patent Publication No. 2005-315883, the reflected light is received while a color of the illumination is changed by time division, and thus it is difficult to acquire the signal of the reflected light associated with the same point, resulting in failing to measure the color with high accuracy.
Further, Japanese Laid-open Patent Publication No. 2010-256324 discloses a technique for acquiring light intensity signals of respective diffracted images on a wavelength band basis with plural pixels of a one-dimensional array sensor wherein the reflected light from the target object is limited spatially with an aperture array such as a pin-hole array, and then the spectrum separation of the reflected light forms plural diffracted images on the one-dimensional array sensor. According to the technique disclosed in Japanese Laid-open Patent Publication No. 2010-256324, plural signals with different spectral characteristics can be acquired at a time and precise spectral characteristics at the respective locations corresponding to the respective apertures can be acquired.
If the target object in the printed image is a user image, for example, it is desirable that the measurement is performed with a high resolution such that colors even in a narrow range can be evaluated. On the other hand, if the target object in the printed image is a color chart, for example, it is desirable that an average color within a range of a color sample is measured with high accuracy but a high spatial resolution is not necessary.
According to the prior art, in order to perform the measurement with a spatial resolution adequate for the target object, it is necessary to replace the aperture array such as a pin-hole array as necessary according to the target object, for example in the case of the technique disclosed in Japanese Laid-open Patent Publication No. 2010-256324.
However, in this case, plural types of the aperture arrays having different intervals between the apertures become necessary, which requires much effort. Further, there may be a concern that a relative positional relationship between diffraction patterns corresponding to the respective apertures of the aperture array and a one-dimensional array sensor is varied by the replacement of the aperture array. If the relative positional relationship is varied, there is a problem that the spectral characteristics of the respective signals are changed and thus the calibration is necessary every time when the aperture array is replaced, thereby complicating the measurements.
The present invention is made in consideration of the matters described above, and an object of the present invention is to provide a spectral characteristic acquiring apparatus capable of acquiring spectral characteristics at plural positions with a spatial resolution adequate for a target object.